1. Field of the Invention
This invention relates to assemblies for supporting read/write heads adjacent rotating disks in disk drives.
2. Background Information
In hard disk drives, magnetic heads read and write data on the surfaces of co-rotating disks that are co-axially mounted on a spindle motor. The magnetically-written “bits” of written information are therefore laid out in concentric circular “tracks” on the surfaces of the disks. The disks must rotate quickly so that the computer user does not have to wait long for a desired bit of information on the disk surface to translate to a position under the head. In modern disk drives, data bits and tracks must be extremely narrow and closely spaced to achieve a high density of information per unit area of the disk surface.
The required small size and close spacing of information bits on the disk surface has consequences on the design of the disk drive device and its mechanical components. Among the most important consequences is that the magnetic transducer on the magnetic head must operate in extremely close proximity to the magnetic surface of the disk. However, because there is relative motion between the disk surface and the head due to the disk rotation and head actuation, continuous contact between the head and disk can lead to tribological failure of the interface. Such tribological failure, known colloquially as a “head crash,” can damage the disk and head, and usually causes data loss. Therefore, the magnetic head is typically designed to be hydrodynamically supported by an extremely thin air bearing so that its magnetic transducer can operate in close proximity to the disk while physical contacts between the head and the disk are minimized or avoided.
The head-disk spacing present during operation of modern disk drives is extremely small—measuring in the tens of nanometers. Obviously, for the head to operate so closely to the disk without excessive contact, the disk must be very smooth and flat. Furthermore, the disk must be made of a stiff material to minimize out-of-plane motion (known as “disk flutter”) induced by surrounding air during rotation and other factors. The total capacity of the disk drive is also enhanced if the disks are made thin, so that more disks (and therefore more data-containing disk surfaces) can be packed into the disk drive. The simultaneous requirements for smoothness, flatness, thinness, and stiffness demand that the disk substrate be made of a high quality material such as pure aluminum. Pure aluminum is sometimes preferable to aluminum alloys because the non-metallic inclusions present in many aluminum alloys can adversely affect disk smoothness after the magnetic coating is deposited thereupon, causing asperities on the surface that later force the head away from the disk. The resulting temporary increases in head-disk spacing cause magnetic read/write errors known as information “bit drop outs”. To ensure an acceptable surface finish, and thereby avoid bit drop outs, designers have often chosen pure aluminum for use as substrates for magnetic disks. For example, U.S. Pat. No. H1,221 discloses the use of pure aluminum rather than an aluminum alloy as a material for magnetic disk substrates, to reduce inclusions that might adversely affect the smoothness of the layers deposited on such substrates, where the deposited layers would cover such inclusions in the disk drive.
Another consequence of the close spacing of information bits and tracks written on the disk surface is that the spindle rotation and head actuator motion must be of very high precision. The microscopic departure of the disk rotation from perfect rotation is often called “run out.” Imprecision or defects in the spindle bearings can lead to spindle run out that does not repeat itself as an integer multiple of the spindle speed (so called “non-synchronous run out”). The head actuator must be actively controlled to follow non-synchronous run out, and accordingly, non-synchronous run out is considered as a disturbance to be minimized. Consequently, spindle bearings must be made of high quality materials to minimize non-synchronous run out over the lifetime of the bearings. If, for example, low grade or impure steel were used in a disk drive spindle bearing, non-metallic inclusions in the steel might cause surface deformities that increase non-synchronous run out. Such non-metallic inclusions might also adversely affect the structural properties of the bearing components, leading to so-called nucleation sites that promote fatigue cracking and shorten bearing life. Accordingly, disk drive designers have used purified steels in disk drive spindle bearings for reasons of surface finish, precision operation, and fatigue strength. For example, U.S. Pat. No. 5,110,687 discloses that inclusions in the aluminum rotor of a spindle in a hard disk drive can cause surface irregularities that can adversely affect precision of rotation. Another U.S. Pat. No. 6,071,358, discloses the use of purified steel having reduced non-metallic inclusions to improve mechanical properties of hard disk drive spindle bearings and thereby increase their life.
The head actuator in modern disk drives must accelerate very quickly to reach information tracks containing desired information, so that the time to access desired information is acceptable to the user. Therefore, the head supporting structure through which a head is mounted to the actuator can not be designed to be too massive. On the other hand, the head supporting structure still must be stiff enough to precisely control the position of the head during operation. Furthermore, the stiffness of the head supporting structure must be sufficient also to limit deflection that might cause contact with the disk during large mechanical shock events occurring under non-operating conditions.
In FIG. 1, an exploded view of three of the components of a magnetic head support structure is shown. The depicted components are the actuator arm 8, suspension spring 12, and swage mount 19. The actuator arm pivot bearing (not shown) facilitates rotation of the actuator arm 8 about axis 6.
Hub 20 of swage mount 19 fits through boss hole 23 in suspension spring 12 and the underside of suspension spring 12 rests on and is spot welded to swage mount 19. Hub 20 also fits through boss hole 22 in actuator arm 8. A swage ball (not shown) of slightly larger diameter than the interior diameter of hub 20 is forced through hub 20 after it has been inserted through boss holes 23 and 22. This “swaging process” causes plastic deformation of hub 20 to a larger diameter, causing it to be “swaged” or to have an interference fit attachment to actuator arm 8. Top surface 18 of suspension spring 12 is then in contact with the under surface of actuator arm 8 near boss hole 22.
Suspension spring 12 is designed to flex out-of-plane in “bend area” region 24 but to resist in-plane deflection. Most suspension spring designs include a region 30 of reduced thickness or removed material, within “bend area” region 24, to enhance out-of-plane flexing or to control resonant behavior of the suspension spring. During manufacture, “bend area” region 24 is deliberately and permanently plastically deformed to include an out-of-plane bend. The purpose of this bend is to impart a pre-load force on the magnetic recording head after the suspension spring 12 is elastically straightened during disk drive assembly. The suspension spring is designed to resist bending of load beam portion 26. The magnetic head (not shown) pivots on dimple 38 but can not translate with respect to dimple 38 because such translation is prevented by a gimbal (not shown) to which the head is glued and that is spot welded to load beam 26. It is through dimple 38 that the pre-load force is transmitted from the load beam 26 to the head.
FIG. 2 shows a top view of swage mount 19 showing the location of view line 3-3. FIG. 3 is a side cross-sectional view of swage mount 19 along the view line 3-3. The cross-hatched region denotes an interior region of the metal (such as 300 series stainless steel) from which the swage mount is made. This interior region would not normally be visible except for an imaginary cross-section made along view line 3-3.
The requirement for a magnetic head supporting structure with low mass but high stiffness has affected the choice of materials used for related sub-components, such as the suspension spring, swage mount, and actuator arm. Typically, suspension springs and swage mounts have been fabricated from stainless steel, whereas the actuator arm has been fabricated of alloys of aluminum, magnesium, and beryllium. Since stiffness and mass are more critical design constraints on these sub-components than fatigue life, the associated material choices heretofore have not been affected by concern for minimizing microscopic inclusions that might form during the processing of the raw materials.
FIG. 4 is an expanded view of region 32 of the cross section shown in FIG. 3. The diagonal cross-hatch lines have been omitted from FIG. 4 in order to conceptually depict material inclusions 42 existing in the metal 40 from which the swage mount 19 is fabricated. Inclusion 44 is depicted to be located on or very near the surface 30 of the swage mount 19.
Remelted metals have never been used to fabricate components of the head supporting structure to improve post-fabrication cleanliness. Past attempts to increase post-fabrication cleanliness of components of the head supporting structure have instead focused on cleaning and/or plating. Disk drive designers believed that microscopic inclusions in the metal from which head supporting structure components are fabricated either remain in the metal or are adequately removed by pre-assembly cleaning.
Uses of purified remelted metals have been restricted to applications where material inclusions would have a significant effect on the component's structural properties (such as surface finish or fatigue life). Remelting processes used in the art to reduce inclusions and thereby improve the structural or surface finish characteristics of mechanical components include vacuum arc remelting (VAR), vacuum induction melting (VIM), and electroslag remelting (ESR). For example, U.S. Pat. No. 6,551,372, U.S. Pat. No. 5,759,303, and U.S. Pat. No. 5,226,946 describe the use of one or more of these remelting processes to improve the structural strength or longevity of gas turbine engine components by reducing inclusions that can lead to nucleation sites, cracks, and fatigue failure. U.S. Pat. No. 5,252,120 describes a surface-finish motivation to reduce inclusions in steel by purification, where the steel is to be used in a “lens-quality” mold.
In contrast, the magnetic head supporting structure is not an application where the use of remelted materials would be justified conventionally by concerns for fatigue strength or surface finish, and accordingly, no motivation has been expressed in the art to use costly remelted metals to fabricate a head supporting structure.
Stainless steels previously used in magnetic head supporting structures have been purified to a lesser extent by a process known as Argon Oxygen Decarburization (AOD). AOD can be used to refine and purify metal so as to ensure improved mechanical properties. In AOD, submerged injectors inject oxygen, argon, nitrogen, and sometimes carbon dioxide, into molten metal heated by an electric arc or coreless induction furnace. Because the blowing is submerged, the gas/metal contact is enhanced, although oxygen is sometimes top-blown over the molten steel. The ratio of oxygen to inert gas injected is decreased as the carbon level in the steel decreases, resulting in efficient carbon removal without excessive oxidation. The primary objectives of the gas injection is to cause a shift in decarburization thermodynamics, remove dissolved gasses (nitrogen and hydrogen), and reduce nonmetallic inclusions. The motivation to reduce the number and size of material inclusions has been to improve the ductility and toughness of the refined steel. Disk drive designers have been satisfied with the ductility and toughness of AOD refined steel and have known no justification to use more costly but purer remelted steels.
However, the present inventors discovered that microscopic inclusions can be released by the metal from which the head supporting structure is made—even after the head supporting structure has been thoroughly cleaned—and the release can be accelerated by disk drive assembly processes that include the deliberate plastic deformation of a portion of the head supporting structure. That is, such deliberate plastic deformation might cause certain material inclusions to be released that cleaning steps would not release. Examples of such processes include the “swaging” process, and the process by which a permanent out-of-plane curvature is deliberately given to the “bend area” of the suspension spring (in the spring's free state) so that it can provide a pre-load force to the head (when the suspension spring is elastically straightened during assembly of the disk drive).
Tribological problems in magnetic disk drives sometimes have non-obvious solutions that, once known and understood, give one disk drive manufacturer a competitive edge over another. Recognizing that released inclusions can comprise microscopic oxide particles that can later contaminate the head-disk interface and ultimately lead to a head crash, the present inventors fabricated novel head supporting structures using remelted metals. Post-manufacture surface microscopy and material analysis of isolated contaminants confirmed the merit of the idea.